Add short vignette on writing your own withr-like functions

[jennybc]

I think this technique is known to some, but not to many others and I can't find an existing demo within withr's docs (I could be wrong). But if I'm right, I think this would make a great short vignette.

Basically showing how to capture and pass along the environment that you want to schedule future actions on.

local_change_digits <- function(new_digits, envir = rlang::caller_env()) {
  op <- options(digits = new_digits)
  withr::defer(options(op), envir = envir)
}

fun <- function() {
  print("inside fun, before calling local_change_digits(): ")
  print(pi)
  
  local_change_digits(2)
  
  print("inside fun, after calling local_change_digits(): ")
  print("after: ")
  print(pi)
  
  invisible()
}

pi
#> [1] 3.141593
fun()
#> [1] "inside fun, before calling local_change_digits(): "
#> [1] 3.141593
#> [1] "inside fun, after calling local_change_digits(): "
#> [1] "after: "
#> [1] 3.1
pi
#> [1] 3.141593

^{Created on 2023-03-16 with reprex v2.0.2.9000}